MIRD pamphlet No. 17: The dosimetry of nonuniform activity distributions -Radionuclide S values at the voxel level

The availability of quantitative three-dimensional in vivo data on radionuc lide distributions within the body makes it possible to calculate the corre sponding nonuniform distribution of radiation absorbed dose in body organs and tissues. This pamphlet emphasizes the utility of the MIRD schema for su ch calculations through the use of radionuclide S values defined at the vox el level. The use of both dose point-kernels and Monte Carte simulation met hods is also discussed. PET and SPECT imaging can provide quantitative acti vity data in voxels of several millimeters on edge. For smaller voxel sizes , accurate data cannot be obtained using present imaging technology. For su bmillimeter dimensions, autoradiographic methods may be used when tissues a re obtained through biopsy or autopsy. Sample S value tabulations for five radionuclides within cubical voxels of 3 mm and 6 mm on edge are given in t he appendices to this pamphlet. These S values may be used to construct thr ee-dimensional dose profiles for nonuniform distributions of radioactivity encountered in therapeutic and diagnostic nuclear medicine. Data are also t abulated for I-131 in 0.1-mm voxels for use in autoradiography. Two example s illustrating the use of voxel S values are given, followed by a discussio n of the use of three-dimensional dose distributions in understanding and p redicting biologic response.